Hmm... well i did my entire exhaust header-back at once but i can say that the header is what creates the rumble. stock/oem manifold rumbles a tiny bit the the borla/replica really rumbles. Catback systems only make it louder. and replacing the stock cats with hfc or none (track pipe) will make it ridiculous loud.
if the op were to put the stock header on and still use the sti catback it would be pretty much the same loudness only it would not rumble/pulsate as much.